There are a ton of buses and they go pretty much everywhere. You can find the bus schedule at http://www.iett.gov.tr/en/. Mostly they are packed beyond belief during rush hours and are complicated by the terrible traffic situation. Luckily, most tourists do not find a need to take a bus during their stay because the entire center of Istanbul is linked by more efficient methods of transportation such as the trams, the metros, and the ferries.
